The present invention relates to a frequency modulator provided with a phase locked loop controlling part.
A phase locked loop (hereinafter will be simply called as PLL) is used to obtain a number of various frequency signals with a good frequency stability from a single reference frequency source. This phase locked loop is often used as a local oscillator in a radio equipment, in recent years. When performing a frequency modulation (FM) on a transmitted carrier, these is often adopted a direct frequency modulating method such that modulating signals are superimposed upon a controlling voltage which controls the output frequency of a voltage-controlled oscillator (hereinafter simply called as VCO) of a PLL.
The following provides a description of a conventional frequency modulator, referring to an accompanying drawing. FIG. 8 is a block diagram of a conventional frequency modulator comprising a PLL.
In FIG. 8, reference numeral 1 denotes a reference signal generator. Used as the reference signal generator is, in general, a generator having a relatively high frequency-stability such as a crystal oscillator or the like. A phase comparator 2 outputs a signal proportional to a phase difference between two signals inputted there to. Reference numeral 3 denotes a low-pass filter, which may be made up of passive elements only, or of an active filter which comprises an active element such as a transistor or the like. Reference numeral 4 is a VCO, the frequency of which is varied in proportion to an input signal voltage, as shown in FIG. 10. A frequency divider 5 divides an input signal to output the divided signal to the phase comparator 2.
Hereinafter will be described an operation of the conventional frequency modulator having the above structure. A part of output signals from the VCO 4 are divided by the frequency divider 5. The phase comparator 2 compares a phase of an output signal from the frequency divider 5 with a phase of a reference signal from the reference signal generator 1. A phase difference signal outputted from the phase comparator 2 is fed back as a control voltage of the VCO 4 through the low-pass filter 3. The control voltage (an output voltage from the low-pass filter) drops, when the phase of the output signal from the divider 5 is leading ahead of the phase of a reference signal from the reference signal generator 1. To the contrary, the control voltage rises, when the phase of the output signal from the divider 5 lags behind the phase of the reference signal from the reference signal generator 1. Such a rise and fall of the control the VCO voltage is effective to control so that a phase difference between the phase of the output signal from the divider 5 and the phase of the reference signal from the reference signal generator 1 may be maintained constant. The phase difference, therefore, becomes constant in a steady-state, and a frequency of the output signal from the divider 5 and a frequency of the reference signal from the reference signal generator 1 are brought into conformity with each other. Accordingly, the following relationship is established: EQU fo=N.multidot.fr
where
fo: an output frequency of the VCO 4; PA0 N: a dividing ratio of the divider 5; and PA0 fr: a reference frequency of the reference signal generator.
An output signal having a frequency fo, which is an integral multiple of the reference frequency fr, may be obtained by varying the dividing ratio N of the divider 5.
In this frequency modulation, a modulating signal from the outside, e.g., an external voice signal, is superimposed upon a controlled input voltage of the VCO 4 so that a frequency generated from the VCO 4 is forced to be varied by the externally inputted signal.
However, the PLL is a negative fed-back circuit to keep the frequency of the VCO 4 constant, as described above. If a gain of the loop of the PLL is sufficiently large and the loop is stable, the frequency is little varied when a modulating signal is superimposed upon the controlled input voltage of the VCO 4. Here, the gain means a gain of an open loop transfer function. This open loop transfer function is the transfer function between a controlled input of the VCO and the output of the low-pass filter 3, when the controlled input of the VCO 4 is cut from the output of the low-pass filter.
Owing to the low pass filter 3, the gain of the transfer function is large in a low frequency range, getting smaller as the frequency becomes higher. A frequency at which a gain of the open loop transfer function is 1 is called a gain crossover frequency. A frequency modulation may therefore be performed on a modulating signal of a frequency equal to or higher than a crossover frequency, having a gain 1 or less. A general radiotelephone is designed to have a gain crossover frequency of about 300 Hz, at which a gain of the open loop transfer function is 1. In such a radiophone, the oscillation frequency of the VCO 4 is modulated in a voice band of about 300 Hz or higher.
To the contrary, a low frequency region cannot be modulated. An affect of a modulating signal added to the VCO 4 returns to an input of the VCO 4 through the divider 5, the phase comparator 2 and the low-pass filter 3. This acts as a negative feedback in a low frequency range, namely, acts to suppress the frequency deviation by the modulation (to suppress it not to depart from the frequency N.multidot.fr), leading thus to quite low modulation sensitivity.
In the gain crossover frequency (in the vicinity of 300 Hz, for example), phase delay due to the low-pass filter 3 operates as positive feedback to emphasize frequency deviation by the modulation, in which case a peak sometimes appears in the vicinity of the gain crossover frequency of the loop, as shown in FIG. 9. It is desirable that the frequency characteristics in modulation sensitivity be flat within a voice signal band. In order to eliminate such a peak in a conventional manner, it is necessary to set the loop gain at a smaller value and to precisely adjust values of respective elements in the low-pass filter 3 to drop the gain crossover frequency, whereby a frequency of the peak may become lower than that of the voice signal band. However, if the gain crossover frequency is dropped, a period of time required to change the frequency is increased. That is, a period of time from changing the dividing ratio of the divider 5 to when the oscillated frequency of the VCO 4 is focused within a range of tolerance deviation of the steady-state value and becomes stabilized, becomes longer. There is thus a disadvantage that a time period required to change the channel in a radiotelephone or the like is increased even if a peak is lowered close to the gain crossover frequency.